This invention relates, in general, to transformer control circuitry, and more particularly to pulse width modulated power supply circuits which utilize output power transformers such as in push-pull output driver circuits.
Pulse width modulated voltage regulators which have output power transformers have several advantages over other types of voltage regulator circuits, one primary advantage being that they utilize transformer coupling to thereby provide output voltage isolation from the unregulated input voltage. Another advantage is that they are more efficient than other types of voltage regulators and are thus used in applications which require relatively large amount of power supply current.
Of particular interest in this application is the output transformer which must pass these large amounts of power to the load. This transformer must transfer the power required without going into saturation. Saturation causes a transformer to lose its transformer action thereby causing the output voltage to drop; and more importantly, saturation of the transformer reduces the back EMF of the transformer to the point where the transformer appears as a short circuit to the driving network. As a result, the driving circuitry, particularly a transistor circuit, can be severely damaged. Preventing the output transformer from going into saturation has been a very troublesome problem in the past and several methods have been employed to prevent this saturation.
The output power transformer is contained within an output driver stage of a pulse width modulated voltage regulator. The driver stage receives pulse width modulated inputs from the voltage regulator and utilizes these inputs to selectively turn on driver transistors which in turn couple an unregulated DC supply voltage to the primary output of the power transformer. Usually, the output driver stage is one of three general types. A first type comprises a push-pull arrangement in which two transistors alternately conduct to gate current through each side of a center tapped primary of the output power transformer. This type of circuit is presented in the principle embodiment of the present application. A second type of circuit consists of a full bridge circuit utilizing four transistors in which two transistors at a time are forced into conduction to allow unregulated power supply current to flow from the positive input through the first transistor, into the primary of the output power transformer, and back through the second transistor to the return of the unregulated DC voltage supply. The direction of the current is reversed when the other two transistors are placed into conduction. The third type of circuit, known as a half bridge circuit, replaces two of the transistors of the full bridge circuit with capacitors.
Of the three types of circuits, the most preferred from an economic standpoint is the push-pull circuit since the full bridge circuit requires twice as many transistors as the push-pull circuit, and the half bridge circuit requires transistors which have twice the current range of the transistors of the push-pull circuit. For high-power applications, for which the pulse width modulator power regulators are best suited, these output transistors are relatively expensive, and their cost is determined to a large extent by the amount of current which they will safely conduct. However, the full bridge and half bridge circuits permit the insertion of expensive capacitors in series with the primary winding of the output power transformer to balance the positive and negative voltages applied to the transformer and thereby significantly reduce the probability of saturation. Thus, circuit designers frequently have selected the half bridge or full bridge circuit together with series capacitors rather than push-pull circuits in order to prevent the saturation of the output power transformer.
Another solution in the past has been to use output power transformers having large air gaps which allow excess magnetic flux density to dissipate between half cycles of the pulse width modulator operation. However, these transformers are both expensive and very large physically and are therefore undesirable. Another method used in the past has been to put inductive chokes in the collectors of the push-pull circuit transistors to current feed the output power transformer and to limit the current to the transformer to prevent the transistors from shorting out. However, these chokes are undesirable in that they require a larger DC unregulated power supply to compensate for the voltage drop across the chokes. Moreover, the chokes themselves are expensive and must be physically large to keep from going into saturation.
Still another method used in the past has been to sense the emitter current of the output driver transistors of the output driver stage and to shut down the regulator when excess current is detected in the emitters. However, this method is unsatisfactory in some cases since, among other reasons, the sensing must be isolated from the regulator in order to preserve the isolation of the unregulated power supply. Also, the amount of time necessary to shut down the regulator is usually several cycles long due to the loop bandwidth of the pulse width modulator, but the shorting mechanism of the transistors caused by the saturation of the output transformer can occur within a single half cycle of the pulse with modulator. Thus, the emitter sensing may be too slow to prevent destruction of the power transistors under certain conditions. Moreover, this method also results in an interruption of the power from the power supply. Finally, sensing the emitter currents provides information about the transformer after it has entered into saturation but does not prevent this saturation from occurring in the first place.
A fourth method of preventing the output tranformer from going into saturation has been to match the positive and negative voltage applied to the transformer by matching the positive and negative characteristics of the pulse width modulator and also by matching the power transistor characteristics. However, this matching has been expensive and generally unsatisfactory since initial room temperature matching does not guarantee matching characteristics over the temperature extremes in which the power supply may operate.
Therefore, it can be appreciated that a symmetry correction circuit which would inhibit saturation of an output power transformer and thereby permit reliable operation of a push-pull type circuit with a pulse width modulated power supply is highly desirable.